1. Field of the Invention
The present invention relates to a lens evaluation method for evaluating resolution of a lens used for a projector, where an image light including test pattern for measuring resolution is irradiated on a screen through a projection lens to display the image of the resolution-measuring test pattern on the screen and the luminance of the displayed test pattern image is detected by an image import device using an image sensor to calculate a resolution evaluation value based on the detected luminance value, and a lens-evaluating apparatus for implementing the evaluation method.
2. Description of the Related Art
Conventionally, a projector including a plurality of liquid crystal panels for modulating a plurality of colors of light in accordance with image information, a cross dichroic prism for combining the color light modulated by the respective liquid crystal panels, and a projection lens for enlarging and projecting the light combined by the prism has been used.
The projection lens used in the projector may have diverse properties such as image resolution, flare and chromatic aberration on account of variation in production process thereof. Since the diversity of the properties of the projection lens exerts influence on the image displayed by the projector, the properties of image resolution, flare and chromatic aberration are evaluated before the lens manufacturer forward the lens and before the lens is assembled into a projector.
Specifically, in order to evaluate, for instance, the resolution of the projection lens, a resolution-measuring test pattern is formed on an evaluation sheet, light is irradiated on the test pattern to introduce an image light including the test pattern into the projection lens, and the image light irradiated by the projection lens is projected on the screen. The image of the test pattern displayed on the screen is detected by an image import device using an image sensor such as a CCD (Charge Coupled Device). The image detected by the device is image-processed by a computer etc. to evaluate the resolution of the projection lens.
In the above, MTF (Modulation Transfer Function) value is generally used as a resolution evaluation value for evaluating the resolution of the lens, which can be obtained according to following formula, where the maximum value of the detected luminance value of the test-pattern image is Imax and the minimum value thereof is Imin.MTF=(Imax−Imin)/(Imax+Imin)
However, in the above arrangement, since measured luminance used for calculating the MTF value calculated by the above formula is a relative value, the MTF value may be varied according to brightness of the image.
Further, in evaluating the resolution of the projection lens, since the image light irradiated from the projection lens to be projected on the screen tends to have the strongest intensity at the central portion to be grown weaker toward the peripheral portion thereof, even when the luminance value is obtained on a plurality of portions of the image projected on the screen, the MTF value cannot be evaluated according to the same standard.
Furthermore, when the brightness of the projected image differ according to the type of the projector, the MTF value of the respective projectors cannot be compared according to the same standard.